DE2310815A1 - PROCESS FOR REGENERATING NITRATION AND CARBON SALT BATHS - Google Patents

Description

GERMAN GOLD AND SILVER SCHEIDEANSTALT FORMERLY IiOESSLER 6 Frankfurt (Main), Weissfrauenstrasse 9

Process for regenerating nitriding and carbonation salt baths

The invention relates to a method for regenerating salt baths, ie molten salt mixtures which are used for nitriding or for carburizing metals.

Such baths essentially consist of mixtures of alkali metal cyanides and alkali metal cyanates as well as alkali metal and alkaline earth metal carbonates and optionally alkali metal chlorides.

For nitriding are as molten baths mixtures of alkali cyan to, AlkaHcyanaton and alkali !! used, which are present as low-viscosity melts at an operating temperature of approx. 370 C. The workpieces are exposed to the action of the melt for a few hours, an air stream being advantageously passed through the melt. By Eind if fundier · η of nitrogen into the workpiece surface to thereby form layers ied from DIFFUN which cause particular ej.no increasing the wear and the Wechselbiegefostigkeit. The bath is depleted of cyanide and cyanate and enriches itself in caibonate, which is ineffective for nitration. In order to maintain its effectiveness, the bath must be replenished with pure alkali metal cyanide for regeneration, a not inconsiderable part of the molten salt being applied each time. These so-called old salts always contain cyanide and are therefore hole-poisonous.

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In addition to alkali metal cyanide, the carbonation baths generally contain barium chloride as the carrier melt and alkali metal carbonate as the active substance. They are operated at a temperature of 800 to 950 C; At this temperature, carbon diffuses preferentially into the surface of the workpieces suspended in the melt for about 1 to 5 hours. Here, too, a gradual conversion of the cyanide into carbonate takes place during the operation of the salt bath, which is ineffective for the carbonation process. The regeneration takes place in a similar way to the nitriding baths by adding cyanide or cyanide-containing salt mixtures. For each regeneration, part of the salt must be removed from the bath and discarded as highly toxic old salt.

Apart from the fact that dealing with the highly toxic cyanide when Approach and during operation of the baths requires special precautionary measures, The detoxification of the old salvage or its removal with subsequent safe depositing means very considerable expenses.

The invention is based on the object of regenerating salt baths for the nitration and / or carbonization of workpieces with the aid of non-toxic additives. It has been found that the baths mentioned can be efficiently regulated by adding polymeric triazine compounds or polymeric hydrogen cyanide. Here I substitute in the molten salt what is present in excess, which is necessary for the nitration between Carbonate which has become ineffective with the polymeric triazines or the polymeric cyanide water is converted into cyanate, with the help of which, depending on the temperature condition (>; un [-; cn , nitration or carbonation takes place. Er.firidmig. ' No compounds that contain cations are added to the bath to be regenerated, so that the structural volume, in contrast to the current work process, is increased during the complete regeneration

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enlarged. So it is not necessary with every regeneration carry out considerable proportions of the molten bath, so that, as a significant advantage of the method according to the invention, none toxic and dangerous waste salts are produced. Since the If molten salt only forms non-toxic cyanate, nitration or carbonization can take place in a cyanide-free system be performed. This in turn has the consequence that when rinsing, those removed from the bath are finished treated Workpieces only disfigure non-toxic wastewater, as this will damage the workpieces Adhering salt residues no longer contain cyanides. This means a considerable relief for the treatment of the hardening wastewater, because, in contrast to the hardening waste water from salt baths regenerated with cyanide, a separate detoxification treatment can now be omitted.

The polymeric triazine compounds to be used according to the invention should be temperature-resistant under normal environmental conditions and do not emit any toxic or dangerous components. You must also without endangering the operating personnel or the Environment in those at temperatures between 500 to 95O ° C Molten salts can be entered. In addition to melamine-formaldehyde resins, polymeric melamines and especially melon have proven themselves touched as a regenerating agent for carrying out the process according to the invention. Melon is a polymerisation product of melamine, which is obtained, for example, by heating melamine to temperatures above 350.degree. C., preferably at 500.degree can be. Melon is completely stable in air up to temperatures above 700 ° C. It settles in those used for nitration Salt mixtures at about 570 C in a calm reaction Development of water vapor around. At 950 C it reacts in salt mixtures for carbonation baths, it is a little more violent than at lower temperatures, but such salt mixtures can be used completely clog in a safe way.

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The process vegetable invention is explained below with reference to some Examples further explained.

Example 1 :

In a cylindrical titanium crucible {16 kg K φ 35 cm, height 70 cm) 6h KCNO kg, 16 kg NaCNO, CO ₂ and h kg Na ₃ CO melted and heated to 570 C. 200 liters per hour are passed through the melt. In 2h hours, the cyanate content falls from hj wt. ^ On ho wt. Fo and the carbonate content increases from 10 wt. ^ O to 13 wt. ^. The initial value can be adjusted by adding 3 ^ g melon. Notched circumferential flexure specimens (oC _v ~ z) and

Treated platelet samples from CK I5 for two hours and then quenched in salt water. When the alternating bending strength was tested on a Schenk-Rapld system, there was an increase in the permanent flexural strength of 0. = 12 kp / inin on ^ T. , = h2 kp / inm. A 18-22 μm thick C-Fe N compound layer was detected by metallographic and X-ray analysis on the platelet probe. A diffusion zone of 0.55 "adjoins the inside of the grain. The thickness of this diffusion zone can be verified with tempered samples (tempering temperature 300 ° C.) on the basis of the needle depth.

Example 2

In a titanium crucible, 60 kg of KCNO, 20 kg of Na CNO, 16 kg of K ₂ CO and h kg of Na _? C0 "melted and heated to 57O ° C. Through the melt 200 1 air / hour. directed. In 2.h hours, the cyanate content falls from about ⁴ h to about ho} Gew.?o wt. ^ O and the carbonate content increases from about IO wt. £ to about 13 Gew.JO. The original word can be set again by adding Ί.8 kg of polymeric hydrogen cyanide .

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Example 3

In a titanium crucible 60 kg KCNO, NaCNO 15 kg, 20 kg of K ₂ CO and 5 kg Na ₃ CO melted and heated to 57o ° C. 200 liters of air / hour are passed through the melt. In Zk hours the cyanate content falls from 41.5% by weight to 38.5 % by weight and the carbonate content increases from 11.5 % by weight to 1.5% by weight. The initial value can be set again by adding 2 kg of polymerized molamine-formaldehyde resin.

Example 4 :

In an iron crucible, 25 kg BaCIp ₁ 65 kg Na _? CO ^ and 10 kg NaCN melted and heated to 930 ° C. To> maintain the bath composition, about 125 g of melon are added per hour. A thin blanket of coal forms on the bath. An additional covering with flake graphite is advantageous. Steel specimens CK 15 get a surface hardness of 64 Rockwell after 2 hours of immersion in the melt and subsequent quenching in salt water. The carbon content is 0.7% C. The duration of the carburization has only a minor influence on the carbon content at treatment times of more than 1 hour. The carburization depth in mm up to 0.3 % C was measured on twisting pins (see table 1)

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Carburization depths in · now up to 0.3 1 2 4th % C 8th hours 0.3-0.5
0.2-0.4
0.1-0.3
0.1> 0.4-0.6
0.3-0.5
0.2-0.4
0.1-0.2 0.6-0.9
0.5-0.7
0.4-0.6
0.2-0.4 6th
1 1.2-1.5
1.0-1.5
0.8-1.0
0.6-0.7 950 °
930 °
900 °
860 °
• i
0.9 - 1.2
0.7 - 1.0
0.5-0.8
0.4-0.6

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Example 5

In an iron crucible 33 kg BaCl _2, 1.75 kg 3.25 kg NaF and melted BaCO and heated to 930 C. A carbonizable bath is obtained by adding 250 g of melon. To maintain the bath composition, 25 g of melon are added per hour. A thin blanket of coal forms on the bath. An additional covering with flake graphite is advantageous. Steel specimens CK 15 obtained a surface hardness of 6Ί Rockwell when immersed in the melt for 2 times and then quenched in salt water. The marginal carbon content is 1 '/' * C.

Example 6

In a cylindrical titanium crucible with a diameter of 35 cm and a height of 70 cm, 30 kg of KCNO, 30 kg of NaCNO, 20 kg of NaCN and 20 kg of KCN are melted and heated to 570 ° C. 150 liters of air per hour are passed through the melt. In 24 hours, the cyanate% increased from the original 35 wt.% To 36 wt ^Ü 4, the cyanide content drops from 18.6 to 17 wt. And it forms 0.6 wt.% Carbonate. The original composition can be restored by adding 0.6 kg of melon. After 90 minutes of treatment in this bath, a 13 - 14 μ thick connection zone of £ -Fe N forms on a CK 15 steel sample.

2 sample ct. »2 is ^, = 40 kp / mm.

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Claims (2)

Claims (1. / Process for regenerating carbonate-containing Molten salt baths for nitriding and / or carbonization of metal workpieces, dadxirch marked, that the regeneration by adding a polymeric triazine compound cder of polymercr hydrogen cyanide satire takes place. 2. The method according to claim I _1, characterized in that ηIs polymeric triazine compounds triaminotriazine-foraldehyde condensation products are used. 3 · Process according to claims 1 and 2, characterized in that as an additive for the regeneration Melon is used. ^l i . Process according to Claim I _1, characterized in that a cyanide-free molten salt is used as the nitriding bath. 5. The method according to claim 1, characterized in that that as ICohDungsbad a molten salt with a maximum cyanide and cyanate content of 3 wt. ^ Is used. 1.3.1973
Dr. Bf-P A0S837 / 0469